The long-term aim of this project is to understand "auditory scene analysis" (ASA), the process whereby the auditory system takes the mixture of sound in complex natural environments and sorts it into packages. of acoustic evidence that probably arose from separate environmental events. This prevents recognition processes from mixing information from different events. The proposed research will study four theoretical aspects of the ASA process: (1) What is the relation between top-down and bottom-up constraints on the perception of speech sounds? (2) Can ASA concepts contribute to an understanding of Massaro's recognition masking" phenomenon? (3) What role do sudden-changes in amplitude play in segregating parts of sound spectra and triggering separate analyses of those parts? (4) How does apparent continuity fit in with other phenomena of ASA? The methodology involves asking human subjects to make judgments of complex sound patterns, answering questions such as "Does the target sound appear in the mixture?", or "Does the target tone pair go up or down in pitch?" While the proposed research is directed toward a theoretical understanding, it would supply information that applicable to three areas of public health. (1) Some people with little or no hearing loss complain of not being able to understand voices-when mixed with other sounds (i.e., they are deficient in ASA). Various types of cues are used to accomplish ASA. Damage to the auditory system may affect the use of different cues to different degrees. Tests based on scientific knowledge of ASA, could assess the patients' residual abilities to use specific kinds of cues for the segregation of signals. This may help in diagnosing the physiological basis of their disorders, and may also permit the fitting of hearing aids that maximize the remaining potential. (2) In the more distant future, "smart" hearing aids may be provided for individuals who have difficulty in segregating concurrent sounds. Computers incorporated in these aids would carry out segregation, based on ASA principles, and allow the user to focus on individual sounds. (3) Knowledge about ASA is also essential in workplace safety where it can contribute to the design of signals that will resist being masked, interfering with one another, or blending with the background.